Marvel has surely regretted their decision to sell some of their characters, and Sony has made them pay at least 4 times and with "The Amazing Spider Man 2" it might add another one.

Spider Man was Marvel's prized character when they held a fire sale for their characters' rights. This allowed studios like Sony and Fox to make movies on their characters. Now that X-Men and Spider Man have become cash cows for their studios with 12 movies made in all (this includes the Wolverine and Fantastic Four movies), the asking price if they want to buy them back would be astronomical.

The only way fans could see them together is through a crossover. However, this is the busiest time ever for all three movie studios, and they are talking of billion dollar stakes.

TASM 2 Producer Avi Arad, who is also a former Marvel Studios executive, is not so hopeful:  "I for one don't see the value for us in it. I think we're doing such a good job with the Spider-Man Universe. Spider-Man is arguably the number one character in the world. He shouldn't make a cameo."

However, he hinted a very slight possibility. "..a natural alliance for some reason...I cannot talk for Sony."

That is true. Sony can enter a joint venture with Marvel. But why would they do that? Also, that question is just the tip of the iceberg. How do they do that?

"The practical side of it is, look, how many movies have we just talked about? It's all we can do to build that universe. We're not going beyond that."

Sony has already announced movies until 2018. They even have release dates of sequels and spin-offs, all designed to make one autonomous and coherent world. Marvel is bent on doing the same, as Screenrant observes: "It's very clear, however, that the Spider-Man, X-Men and Avengers movie universes are heading in very different directions. A crossover on this scale could happen in some form, eventually, but don't expect it anytime soon."

Perhaps after the first Avengers, when they were still in New York, but Kevin Feige of Marvel has made it very clear that they want more of the "World" Avengers and beyond.